An Eddy-Current Testing Method for Measuring the Thickness of Metallic Plates

Thickness measurements of metallic plates are mandatory in many industrial scenarios. Methods based on eddy-current testing (ECT) are ideal for fast and accurate online contactless thickness measurements, making them very attractive in the Industry 4.0 scenario. This contribution is focused on a specific and robust ECT technique proposed in the past by the scientific community. The main limitation is its applicability to thin materials only, where the thickness of the material is much smaller than the overall size of the ECT probe. Extending the range of applicability to thicker materials introduces a progressive and severe degradation of the measurement accuracy. In this article, we analyze the theoretical foundation of this method with an entirely original approach based on the celebrated Buckingham π theorem. In doing this, we draw the complete theoretical picture of the method, providing a simple, clear, and rigorous view of its performance and intrinsic limitations. Moreover, we propose two solutions, one analytical and the other iterative, to accurately estimate the thickness of the materials from thin to thick values. Finally, a numerical analysis combined with an experimental campaign confirms the effectiveness of the proposed solutions, making the method suitable for industrial and other applications

Magnetic Flux Leakage techniques for detecting corrosion of pipes

Oil and gas pipelines are subjected to corrosion due to harsh environmental conditions as in refinery and thermal power plants. Interesting problems such as internal and external corrosion, emerging from the increasing demand for pipeline protection have prompted this study. Thus, early detection of faults in pipes is essential to avoid disastrous outcomes. The research work presented in this thesis comprises investigations into the use of magnetic flux leakage (MFL) testing for pipe in extreme (underwater and high temperature) conditions. The design of a coil sensor (ferrite core with coil) with a magnetic circuit is carried out for high temperature conditions. The sensor thus developed lays the ground for non-destructive evaluation (NDE) of flaws in pipes through the MFL technique. The research focusses on the detection and characterization of MFL distribution caused by the loss of metal in ferromagnetic steel pipes. Experimental verifications are initially conducted with deeply rusted pipe samples of varying thicknesses in air. AlNiCo magnets are used along with Giant Magneto Resistance (GMR) sensor (AA002-02). The experiment is further repeated for saltwater conditions in relation to varying electrical conductivity with radio frequency identification (RFID) technique. A further study carried out in the research is the correlation between magnetic and underwater data communication. The study has resulted in the development and experimental evaluation of a coil sensor with its magnetic response at room and high temperatures. This makes the system effective under high temperature conditions where corrosion metal loss needs to be determined

Magnetic Flux Leakage techniques for detecting corrosion of pipes

Oil and gas pipelines are subjected to corrosion due to harsh environmental conditions as in refinery and thermal power plants. Interesting problems such as internal and external corrosion, emerging from the increasing demand for pipeline protection have prompted this study. Thus, early detection of faults in pipes is essential to avoid disastrous outcomes. The research work presented in this thesis comprises investigations into the use of magnetic flux leakage (MFL) testing for pipe in extreme (underwater and high temperature) conditions. The design of a coil sensor (ferrite core with coil) with a magnetic circuit is carried out for high temperature conditions. The sensor thus developed lays the ground for non-destructive evaluation (NDE) of flaws in pipes through the MFL technique. The research focusses on the detection and characterization of MFL distribution caused by the loss of metal in ferromagnetic steel pipes. Experimental verifications are initially conducted with deeply rusted pipe samples of varying thicknesses in air. AlNiCo magnets are used along with Giant Magneto Resistance (GMR) sensor (AA002-02). The experiment is further repeated for saltwater conditions in relation to varying electrical conductivity with radio frequency identification (RFID) technique. A further study carried out in the research is the correlation between magnetic and underwater data communication. The study has resulted in the development and experimental evaluation of a coil sensor with its magnetic response at room and high temperatures. This makes the system effective under high temperature conditions where corrosion metal loss needs to be determined

Investigation of the factors influencing magnetic flux leakage and magnetic Barkhausen noise

Magnetic Nondestructive methods, including Magnetic Flux Leakage (MFL) and Magnetic Barkhausen Noise (MBN), are widely used to evaluate the structural integrity, mechanical properties, and microstructures of ferromagnetic materials. The MFL method is commonly applied to nondestructively evaluate the damage in ferromagnetic materials due to its reliability, high efficiency, and cost-saving. The MBN method is applicable in nondestructive evaluation (NDE) of mechanical and material properties due to the high sensitivity of Barkhausen jumps to residual (or applied) stress and microstructure of ferromagnetic material. The recognized research and successful applications helped these methods to be feasible NDE tools. However, there are still several important factors that may have noticeable influences on the experimental results of these NDE methods and usually are ignored in applications. In this thesis, the effects of the factors of stress and temperature on the MFL method, as well as the influences of temperature and microstructure on the MBN method are analysed via analytical and numerical modelling. A new finite element model for evaluating the effect of stress on the MFL amplitude is proposed and validated in defective steel under various stresses. Moreover, the new models describing the direct effect of temperature and the combined effects of temperature and thermal stress on the MFL signals are presented. The direct and combined effects are verified in an environmental temperature range from -40℃ to 60℃ by experimental results of a single lamination steel and multilayer structure, respectively. A set of newly derived equations modelling the effect of temperature on the MBN signals are given. Both the direct effect of temperature and the combined effects of temperature and thermal stress are considered in these equations, which are further simplified to linear functions consistent with the measured results in an environmental temperature range from -40℃ to 40℃. Furthermore, the microstructure factors, including the microstructure induced anisotropy in non-oriented silicon steel and the metallographic phases changing with carbon content in steel, are theoretically and experimentally investigated, respectively. For the factor of anisotropy, a new model II describing the dependency of Barkhausen emission on the angle between measurement and rolling directions is proposed. It allows the deduction of a trigonometric function to evaluate the effect of directional anisotropy. The agreement of simulated and measured results of MBN signals indicates the feasibility of the presented model. In the investigation of the influence of carbon content in steel on MBN signals, an optimisation method for MBN pick-up coil is proposed, and a multifunctional measurement system is presented. The correlations of the MBN signals and hysteresis loops related to the carbon content in steel are experimentally observed. The method for the quantitative evaluation of the carbon content using MBN signals and hysteresis loops are discusse

Modelling and experimental investigation of magnetic flux leakage distribution for hairline crack detection and characterization

The Magnetic Flux Leakage (MFL) method is a well-established branch of electromagnetic Non-Destructive Evaluation (NDE) extensively used to assess the physical condition of ferromagnetic structures. The main research objective of this research work presented in this thesis is the detection and characterization of the MFL distribution caused by rectangular surface and far-surface hairline cracks. It looks at the use of the direct current and pulsed current techniques to investigate the presence of hairline cracks in ferromagnetic steel pipelines, by comparing the Finite Element Modelling (FEM) technique with practical experiments. First, the expected response of an MFL probe scanned across the area of a hairline crack was predicted using the 3D FEM numerical simulation technique. The axial magnetization technique is employed and the characteristics of the surface and far-surface leakage field profile